As the performance and functions of computers and communication devices have improved and become increasingly network-oriented in recent years, signals trend toward higher frequencies used for the high-speed transmission of large volumes of information.
Printed wiring boards are used in these communication devices. A printed wiring board is usually produced by first producing a copper-clad laminate by heating and pressing a substrate made of a resin or other insulating material and a copper foil that serves as the conductive part, then etching to form a circuit. The process is completed by mounting semiconductor elements or the like on this circuit.
Copper foils include rolled copper foil and electrolytic copper foil, and the surface treatment method differs with the type of foil used. However, the majority of the copper foil used in printed wiring boards is electrolytic copper foil because of its good adhesive strength with insulating substrates. An electrolytic copper foil is formed by casting an electrolyte composed of a sulfuric acid acidic copper sulfate solution between the drum of a cathode made of titanium or the like, and an anode disposed across from this cathode, and electrodepositing copper onto the drum. The copper foil is formed by passing a current between the revolving drum (cathode) and the anode and continuously peeling away the copper foil (called a “raw” foil) that has been electrodeposited in a specific thickness on the drum surface. The drum side of the electrolytic copper foil is relatively smooth and is called the S side (the shiny side), while the other side has more surface roughness and is called the M side (the matte side).
Because of their excellent adhesion characteristics, epoxy resins have been used fairly widely for the insulating materials of printed wiring boards. However, epoxy resins generally have a high dielectric constant or dielectric tangent and thus unsatisfactory high frequency characteristics. A resin with excellent dielectric characteristics has to be used, but resins with a low dielectric constant or dielectric tangent tend to exhibit poor adhesion characteristics, owing to having only a few functional groups with a high polarity that contribute to adhesion. Also, the copper foil of a copper-clad laminate for higher frequency applications preferably has as little surface roughness as possible. The reason this lower profile is preferable for the copper foil is theorized that current flow becomes concentrated in the surface of the copper foil as the frequency rises (this is called the skin effect), meaning that the surface roughness of a copper foil greatly affects transmission loss.
One of the most important characteristics required of a copper foil is its adhesion to the insulating resin that makes up the printed wiring board. To improve this characteristic, with an electrolytic copper foil, large undulations are generally provided to the M side at the raw foil stage, and roughening particles are provided on the M side in treatment working, thereby, the adhesive strength is enhanced through a mechanical effect (anchor effect). The roughening particles used in the roughening treatment come in all different sizes, but are usually nodule- or needle-shaped particles made of copper or copper oxide and measuring about 0.2 to 3 μm in overall length. However, as mentioned above, the patterns of printed wiring have been made increasingly finer in recent years, so there is a need for a copper foil of lower profile. This requirement has been dealt with in the past, for example, by reducing the size of the undulations of the copper foil at the raw foil stage, or by changing the shape of the roughening particles (making them finer).
Standard means employed to improve the adhesion of a copper foil with an insulating resin are the surface treatment of a copper foil with a silane coupling agent, or the addition of a silane coupling agent to the resin. While the undulations of the copper foil tend to be made as small as possible in the manufacture of a copper foil for a copper-clad laminate for higher frequency applications as above, after the roughening particles have been applied, an anticorrosive treatment and silane coupling agent treatment are currently performed. Nevertheless, since these roughening particles increase the surface roughness and surface area of a copper foil, they are considered to have an adverse effect on transmission characteristics.
In the case of a copper foil for a multi-layer printed circuit inner layer, the following approach has also been taken. After depositing roughening particles on the M side, this side of a copper foil used for an inner layer of a multi-layer board is generally subjected to an anticorrosive treatment and a silane coupling agent treatment. Next, the foil is put together with the inner layer substrate, and then a circuit is etched. Furthermore, the S side of the copper foil is subjected to a black oxide treatment, after which the outermost layer is stuck. However, not only managing the liquid in the black oxide treatment is difficult, but insulation characteristics and interlayer connection reliability tend to suffer because of frequent haloing at the subsequent through-hole production stage. Furthermore, chemical reduction treatments have been performed in an effort to prevent the haloing, but this drives up the cost. The use of a copper foil treated on both sides (double-treated foil) has also been discussed in Japanese Patent Publication H8-222857 as another option. With this method, though, not only are more copper foil manufacturing steps entailed, but the S side of the copper foil is the side on which etching is commenced, because the M side of the double-treated foil is used as the side to which the inner layer substrate is bonded. Accordingly, the etching factor is not very high, and it is difficult to accommodate fine, high-density circuits. Another recent technique is to provide roughening particles to the S side of a copper foil, and apply the inner layer substrate to the S side, as discussed in Japanese Patent Publication H8-511654.